Convection ovens circulate heated air in a cooking chamber to distribute heated process air evenly around food product. Some convection ovens generate heat using a flame element. Previously implemented convection ovens using the flame element are typically one of two types: direct-fired or indirect-fired. In direct-fired ovens, the products of combustion produced by the flame element may be vented directly into the process airflow and come in contact with the food in the cooking chamber. In indirect-fired ovens, the products of combustion may be separated from the process airflow and do not contact the food in the cooking chamber. Indirect firing of a convection oven may be preferred in ovens where control of the amount of moisture in the airflow is critical to the quality of the cooking process. The addition of combustion exhaust by a direct-fired method may increase or decrease moisture content and thereby alter the baking or cooking process.
There is an inherent inefficiency in any fuel-fired process due to the need for oxygen in the combustion process. Oxygen is nearly always supplied by ambient air, which may be at or around ambient temperature. A portion of the energy supplied by the fuel is utilized to heat the oxygen and the associated air, which is mostly inert in the combustion process. Inefficiency is compounded when the process temperatures are elevated, such as in baking and cooking ovens where the process air may be between 150° C. and 250° C. above ambient temperature. In such cases, inefficiency is at least partially a result of the energy required to heat air used for combustion from ambient temperature to a temperature above the desired cooking process temperature.
Previously implemented solutions have addressed the issue of combustion inefficiency by improving the effectiveness of the heat transfer from the combustion process to the process air. Some solutions have used “crossflow” where the flame, products of combustion and exhaust are ducted transversely to the flow of process air to increase the heat transfer efficiency. Other solutions have used “cross counter-flow” where the flow of flame, products of combustion and exhaust are ducted transversely to the flow of process air in successive passes progressing in a direction counter to the flow of process air. Although the previously implemented solutions have achieved some degree of success, they have failed to fully overcome the physical limitations of space within the conventional size of an oven and the associated cost and difficulty of construction.